1. Field of the Invention
The present invention relates to an exhaust gas recirculation device, which has an exhaust gas recirculation control valve that opens and closes an exhaust gas recirculation passage, and, more particularly to the exhaust gas recirculation device, which employs a butterfly valve as a valve body of a motor-driven exhaust gas recirculation control valve.
2. Description of Related Art
An exhaust gas recirculation device, which decreases a maximum temperature of combustion, and which reduces harmful substances (e.g., nitrogen oxides) contained in an exhaust gas, has been known. In the above exhaust gas recirculation device, an exhaust recirculation gas (an EGR gas), which is a part of the exhaust gas that flows inside an exhaust pipe of an internal-combustion engine, is mixed into an intake air that flows inside an intake pipe. However, the recirculation (reflux) of the exhaust gas toward an intake side involves deteriorating an output of the internal-combustion engine and performance of the internal-combustion engine. Thus, a flow rate of the exhaust gas (an exhaust gas recirculation quantity: an EGR quantity) that recirculates from the exhaust pipe to the intake pipe needs to be adjusted. Accordingly, the exhaust gas recirculation device, in which an exhaust gas recirculation pipe (an EGR pipe) is provided with an exhaust gas recirculation control valve (an EGR control valve), has been publicly known. More specifically, with regard to the above exhaust gas recirculation device, the part of the exhaust gas of the internal-combustion engine recirculates from an exhaust path through the exhaust gas recirculation pipe into an intake path. Furthermore, the exhaust gas recirculation control valve adjusts an opening area of an exhaust gas recirculation passage, which is formed inside the exhaust gas recirculation pipe.
With reference to FIGS. 10 to 13, an example of structures of the above exhaust gas recirculation control valves will be described below. Rotational motion of an output shaft 102 of an electric motor 101 is transmitted to a valve shaft 104 via a speed reducing gear mechanism 103. A butterfly valve 105 is held and secured to an axial end of the valve shaft 104. By rotating the butterfly valve 105 about a rotational axis of the valve shaft 104, an exhaust gas recirculation passage 111, from which the EGR gas flows into an inside of a housing 106, is opened and closed (for example, refer to U.S. Pat. No. 6,135,415 and EP-1102929-B1). A mixing chamber 112, an air suction passage 110, and an air delivery passage 113 are included inside the housing 106. In the mixing chamber 112, the exhaust gas that flows from the exhaust gas recirculation passage 111 is mixed into the intake air, which is suctioned into the internal-combustion engine. The intake air flows through the air suction passage 110 into an inside of the mixing chamber 112. The intake air flows from the mixing chamber 112 through the air delivery passage 113 toward an inlet port of the internal-combustion engine. The air suction passage 110, the mixing chamber 112, and the air delivery passage 113 constitute a part of an intake path of the internal-combustion engine. Additionally, an EGR gas recirculation opening 114 opens in an inner wall surface of the mixing chamber 112. A valve bearing part 115 rotatably holds the valve shaft 104 via bearing parts 116,117.
The motor-driven exhaust gas recirculation control valve described above employs a water cooling structure or an inlet air cooling structure. The water cooling structure cools down the electric motor 101 and the like by utilizing engine coolant. The inlet air cooling structure cools down the electric motor 101 and the like by using the intake air that flows inside an intake air passage (the intake path). As a result, a temperature of the electric motor 101, the speed reducing gear mechanism 103, the valve shaft 104, or the valve bearing part 115 does not exceed an allowable heat-resistant temperature due to heat conduction from the EGR gas. In addition, the water cooling structure involves forming a coolant path in the housing 106, and drawing the engine coolant from a coolant circuit on a vehicle side. The inlet air cooling structure has a simple structure, and does not require the water cooling.
However, in the conventional motor-driven exhaust gas recirculation control valve, the electric motor 101, the speed reducing gear mechanism 103 and the valve shaft 104 are coaxial along an axis, which is perpendicular to a central axis of the intake path (i.e., the air suction passage 110, the mixing chamber 112, and the air delivery passage 113) of the internal-combustion engine, as shown in FIGS. 10 and 12. The high-temperature EGR gas flows from the exhaust gas recirculation passage 111 through the EGR gas recirculation opening 114 into the inside of the mixing chamber 112. The electric motor 101, the speed reducing gear mechanism 103, and the bearing part 116 are placed at or around a portion of the inner wall surface 119 of the mixing chamber 112, to which the high-temperature EGR gas is directed from the exhaust gas recirculation passage 111. The bearing part 117 is placed near the EGR gas recirculation opening 114.
For this reason, it is possible that the above high-temperature EGR gas contacts the inner wall surface 119 of the mixing chamber 112 before it is sufficiently mixed with the intake air that flows from the air suction passage 110 into the inside of the mixing chamber 112 to reduce its temperature. The contact of the high-temperature EGR gas with the inner wall surface 119 facilitates the conduction of heat of the high-temperature EGR gas to the electric motor 101, the speed reducing gear mechanism 103, and the bearing part 116 via the housing 106, thereby hindering efficient cooling of the electric motor 101 and the like.